Re: [patch] multipath RAID personality, 2.4.10-pre9

[Andreas Dilger <adilger@turbolabs.com>]

On Sep 14, 2001  12:01 +0200, Ingo Molnar wrote:
> the attached patches implement multipath IO for Linux in form of a sw-RAID
> personality. Multipath-IO is the ability of certain devices to address the
> same physical disk over multiple 'IO paths'.

> /*
>  * This routine returns the disk from which the requested read should
>  * be done. It bookkeeps the last read position for every disk
>  * in array and when new read requests come, the disk which last
>  * position is nearest to the request, is chosen.

I'm not sure I understand why this is here?  If we are talking about a
multipath situation, there IS only a single disk, so which path is chosen
is mostly irrelevant.  Also, it is my understanding that with some multipath
hardware, if you read from the "backup" path it will kill access to the
primary path (this can be used when more than one system access shared disk
for failover).  As a result, we should always read from the "primary" path
for each disk unless there is an error.

Cheers, Andreas
-- 
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert